Sealing of colored anodized aluminum

ABSTRACT

A process for treating anodized aluminum colored with a dye which is insoluble in water comprising sealing the colored anodized surface in a strong solution of a neutral salt in water, which solution is held at or near its boiling point. Advantageously sodium sulfate is used as the neutral salt.

United States Patent Deltombe us] 3,652,,M9 [451 Mar. 28, W72

[54] SEALING OF COLORED ANODIZED ALUMINUM [52] U.S. Cl. ..204/35 N, l48/6.1 [S I] Int. Cl. ..C23l 5/04 [58] Field ofSearch ..204/35, 35.1, 38 A; l48/6.l

[56] References Cited UNITED STATES PATENTS 3,172,786 3/1965 Kirby ..148/6.l

FOREIGN PATENTS OR APPLICATIONS 854,932 4/1940 France OTI-Il-IR PUQLICATIONS Wernick et al.; The Surface Treatment and Finishing of Aluminum and its Alloys; Robert Draper, Ltd. Tcddinmon I959; 2nd Edition; page 386 Primary Examiner-John I-l. Mack Assistant Examiner-William I. Solomon Attomey-Robert E. lsner, Peter J. Franco and Spencer T. Smith [57] ABSTRACT A process for treating anodized aluminum colored with a dye which is insoluble in water comprising sealing the colored anodized surface in a strong solution of a neutral salt in water,

which solution is held at or near its boiling point. Advantageously sodium sulfate is used as the neutral salt.

5 Clalms, No Drawings SEALING F COLORED ANODIZED ALUMINUM The present invention is concerned with sealing anodized aluminum and, more particularly, with sealing anodized aluminum which has been colored by means of water-insoluble organic dyes.

It is known to produce oxidic coatings on aluminum (including for purposes of this specification and claims alloys rich in aluminum, i.e., containing more than about 80 percent aluminum) by means of anodic reactions. For example, by making an aluminum body an anode in an electrolyte consisting of a percent by volume aqueous solution of sulfuric acid and passing electric current therethrough at an anode current density of about 15' amperes per square foot (a.s.f.) while maintaining, the electrolyte at about 70 F. (21 C.), one can produce an oxidic layer up to about 1 mil thick in about 1 hour or less. As produced, this oxidic layer is both soft and porous and provides little protection to the underlying base metal. However, if the oxidic layer is treated with boiling water or various aqueous solutions at high temperatures, e.g., 180 F. to 215 F., it transforms by chemical reaction into a hard, usually transparent, non-porous layer which provides excellent protection to the underlying metal. This latter treatment is called sealing.

A number of different ways to seal uncolored anodized aluminum with water have been suggested in the past. The anodized aluminum can be immersed in pure or slightly acidic water maintained at its boiling point for to minutes or so. The anodized aluminum can be exposed to superheated steam for periods of time up to A hour and more. Baths containing about 0.25 to about 2 percent or more by weight of nickel or cobalt acetate in water which are maintained at a temperature of about 90 C. can effectively seal anodized aluminum at least in part through precipitation of hydrolysis products in the pores of the anodic coating. Solutions containing about 20 percent by weight of sodium sulfate in water have been found to give rapid and effective sealing of anodized aluminum when used at their boiling points.

It is also known to color anodized aluminum by dyeing the freshly formed porous anodized surface with organic dyes prior to sealing. Dyeing can take many forms. It is known to dye anodized aluminum from aqueous solutions, from organic solutions, with ink deposits and from heat transfer devices. In my copending application, Ser. No. 599,543 filed on Dec. 6, 1966, it is disclosed that anodized aluminum can be colored with vaporized dyes. After dyeing is completed, the colored anodized aluminum must be sealed in order to prevent accidental staining, and to harden the surface and thus protect the underlying aluminum. During sealing, however, it is necessary to guard against loss of color strength in the colored surface.

For all practical purposes solvent soluble dyes such as Orasol Yellow 3Sudan Yellow GR, Oracet Blue B, and Oil Blue NS are totally insoluble in water at room temperature. Their chemical structures do not contain sufficient hydrophilic groups to provide any magnitude of water solubility. Strangely, however, these dyes bleed severely in boiling water used to seal anodized aluminum. It is thought that such bleeding is due to the fact that at or near the temperature of boiling water the dyes are in act water soluble to an appreciable extent.

[t is an object of the invention to provide a novel sealing process for anodized aluminum colored with water-insoluble organic dyes.

Another object of the present invention is to provide a novel process for the protection and decoration of aluminum.

Other objects and advantages of the present invention will become apparent from the following description.

Generally speaking, the present invention contemplates the sisting of sodium and potassium sulfates, sodium and potassium chloride, and sodium chlorate, said aqueous solution being held at a temperature of at least about 90 C.

The utility of the present process in effectively sealing anodized aluminum while, at the same time, inhibiting the bleed of organic dyes, insoluble in water at room temperature, into the sealing bath is surprising. it has been known to seal uncolored anodized aluminum with boiling solutions of sodium sulfate in order to accelerate the sealing thereof. However, when sodium sulfate solutions have been used to seal anodized aluminum colored by means of dyes dissolved in aqueous baths, the color bleeds into the sealing bath. Apparently, the art has found not only that sodium sulfate solutions will have this effect, but also that solutions of other salts will also accelerate bleed of water-soluble dyes. In 1963, a reputable European supplier of water-soluble dyes for coloring anodized aluminum advised that:

Sealing baths which have been in use a long time may contain accumulation of salts which will aggravate bleeding of dyeings and it is therefore recommended that sealing solutions be renewed periodically." If, as is believed, dyes which are insoluble in water at room temperature become soluble in water at or near 100 C., prior experience would indicate that use of sodium sulfate for sealing would aggravate dye bleeding.

On the contrary, the present process is highly advantageous in that it combines decorating anodized aluminum with a wide variety of dyes in a wide variety of manners with a sealing operation which is specifically adapted to cooperate with the decorating step so as to retain the color intensity, luminescence, and brilliance produced in the decorating step. In the process of the present invention, it is advantageous to employ sodium sulfate as the neutral salt in solution concentrations of about 15 percent by weight (based on anhydrous sodium sulfate content) up to saturation. Sodium sulfate as the anhydrous salt or as the decahydrate (Glaubers Salt) is a readily available chemical which is sufficiently cheap such that if the sealing bath accidentally becomes contaminated relatively little monetary loss is involved in discarding it. Its solubility is such that even at temperatures approaching 0 C. a substantial amount of the salt will remain in solution. All in all, the advantages of using sodium sulfate are such that even though other neutral salts are operative for purposes of the present invention, practicalities preclude their use except under special circumstances.

In carrying the invention into practice, one can anodically treat an aluminum surface in any manner which will produce a porous, oxidic surface. As discussed hereinbefore, anodizing in dilute sulfuric acid at about room temperature is satisfactory. Other anodizing electrolytes, for example, aqueous solutions of oxalic acid, mixtures of oxalic acid and sulfuric acid, chromic acid and the like can also be used. One should avoid anodizing in electrolytes such as aqueous solutions of boric acid since the surface film produced by such anodizing is usually not sufficiently porous to permit dye penetration. After anodizing is complete, the anodized surface should be rinsed in water and allowed to dry. Decorating is advantageously conducted within a short time after anodizing since exposure of a freshly anodized surface to moist air comprises a natural, slow, sealing process ultimately resulting in a sealed surface which will not accept dye.

Decorating can be done in many different ways providing, of course, that non-water-soluble organic dyes are employed. Such dyes are often of greater color strength, brilliance, light fastness and oxidation resistance than water-soluble dyes. One of the simplest ways of using a non-water-soluble dye to decorate anodized aluminum is to dissolve it in an organic liquid which is a solvent for it. For example, liquids such as keytones, alcohols, esters, aliphatic and aromatic hydrocarbons, dimethylsulfoxide, dimethylacetamide, dimethylformamide and others can be used depending upon the solubility of the dye.

Another method in which organic dyes, not soluble in water, can be used to dye anodized aluminum comprises compounding the dye into an ink and applying the ink by printing, screening or spraying onto selected portions of an anodized surface. The ink may then be allowed to rest on the porous 5 surface for a period of time or may be gently heated so as to pennit dye molecules to permeate the anodized surface under the ink deposit. Generally speaking, the pores in the anodized surface are sufficiently small such that binder materials in the ink, e.g., varnish resins and the like, do not readily penetrate. After sealing in accordance with the teachings of the present invention, the residual ink deposit can be wiped from the surface with the use of a solvent such as methyl ethyl ketone (M.E.K.) l 5 Another way of using organic dyes not soluble in water to color anodized aluminum is to employ them in the process as described and claimed in U.S. application Ser. No. 570,593 now U.S. Pat. No. 3,484,342 by Carl B. Blake and Adolph Fuerst, which application is assigned to the assignee of the present application. Briefly, the Blake et al. application describes a process wherein ink containing a sublimable organic dye (usually insoluble in water) is printed on a paper sheet to form a pattern. The paper sheet is then placed adjacent an anodized surface and heat is applied to transfer the dye by sublimation from the ink deposit into the pores of the anodized surface. After heating is discontinued, usually after less than a minute, the anodized aluminum surface is then sealed in accordance with the present invention.

Many organic dyes are usable in accordance with the present invention. The operable organic dyes are characterized by lack of solubility in water. For purposes of this specification and claims, if a dye does not bleed color into water when in contact therewith at room temperature, it is to be considered insoluble in water. Examples of dyes which can be used in the aforestated ways to color anodized aluminum, after which dyeing, the anodized aluminum is advantageously sealed in neutral salt solutions are included in the following Table:

TABLE Colour Index No.

Solvent Yellow 17 (12770). Solvent Yellow 30 (21240). Solvent Yellow 33 (47000). Solvent Red 16.

Solvent Blue 19.

Solvent Blue 61.

Disperse Red 59.

Source CIBA G CCC CIBA CIBA CCC DuP Commercial Designation Orasol Yellow 3G Sudan Yellow GR Oil Yellow SIS.

Latyl Cerise B Resoforrn Blue 3GL. G Sudan Blue CSP G Oracet Violet B CIBA Solvent Violet 12. 5O Splat-g Soluble Fast Blue NAC Solvent Blue 39. l

F. Orasol Red 2B CIBA Solvent Red 9.

Orasol Scarlet 213.. CIBA Solvent Red 50.

Nora:

CIBA=Ciba Chemical and Dye Co., Div. of Clba Corp. G= General Aniline and Film Corp. CCC=American Cyanamid 00., Dyes Dept. DuP=E.I. du Pont de Nemours & Co. Inc. NAC=Allied Chemical Corp., National Aniline Division.

ples were given:

EXAMPLEI A sheet of aluminum alloy is anodized in a 15 percent sulfuric acid electrolyte to produce an anodic film about 0.5 mil thick. The anodized sheet is rinsed in water and dried. Coloration of the anodized sheet is accomplished by dipping in a solution of Latyl Cerise B dissolved in toluene. After dipping, excess solvent on the anodized and colored sheet is allowed to evaporate. Subsequent thereto, the anodized and colored sheet is sealed in a boiling 20 percent solution of sodium sulfate in water. No substantial bleed occurs and the sealing bath remains uncolored. A similarly treated sheet sealed in boiling water bleeds severely.

EXAMPLE ll A sheet of aluminum anodized, rinsed and dried in the same manner as the alloy sheet of example I is colored by the heat transfer process described in the Blake and Fuerst U.S. application Ser. No. 570,593, now U.S. Pat. No. 3,484,342. In this heat transfer process the dye Sudan Yellow GR is employed as the coloring agent in the ink deposited on the transfer sheet. After transfer, the anodized aluminum sheet exhibits a yellow design. Sealing in an aqueous, boiling 20 percent solution of sodium sulfate preserves the clarity of the design and keeps the sealing bath uncolored. Sealing in boiling water results in substantial fading and coloration of the sealing water.

EXAMPLE lll An image is silk-screened onto two anodized, rinsed and dried aluminum sheets using a silk-screen ink containing Sudan Blue CSP as the coloring agent in an ethyl cellulose-cellulose acetate-butyl acetate vehicle. After drying, assisted by gentle heat, one of the decorated sheets is sealed in a 20 percent solution of sodium sulfate in water and the other of said decorated sheets is sealed in water, both sealing agents being used at the boiling point. Thereafter, excess ink vehicle is removed from the sealed surfaces. The sheet sealed in sodium sulfate solution has a design which is substantially deeper in shade, sharper and more brilliant than the design on the sheet sealed in water. The sulfate sealing bath remains uncolored whereas the water sealing bath is colored blue.

EXAMPLE IV Prints were made on duplicate, porous anodized aluminum sheets by means of the silk-screen process using a silk-screen ink having a vehicle comprising ethyl cellulose, cellosolve acetate and butyl acetate and having dispersed and/or dissolved therein Orasol Yellow 3R, Orasol Scarlet 2B and Oracet Blue B. The prints were black in color. Upon sealing in boiling water, the sealing bath turned yellow. After removing excess ink vehicle, definition of lines in the print was found to be poor because of yellow outrun. Slight yellow smearing occurred. On the contrary, after sealing in an aqueous solution containing 20 percent by weight of sodium sulfate held at its boiling point, there was no bleed into the sealing bath. After removal of the excess ink vehicle, definition of lines in the print was found to be sharp. There was no smearing.

EXAMPLE V A printed image is applied to two anodized, rinsed and dried aluminum sheets by gravure printing using a gravure ink containing Orasol Red 28 dispersed and/or dissolved in a vehicle containing ethyl cellulose, gum rosin and a mixture of alcohol, ester and ketone solvents. The sheets were sealed, one with boiling water, the other with boiling 20 percent solution of sodium sulfate in water, and excess ink vehicle removed. In both instances the prints were sharp with a strong, bright bluish-red color and were not smeared. During sealing, however, the dye bled heavily into the water sealing bath whereas there was no bleed into the sulfate solution bath.

The process of the present invention is highly advantageous since it permits the use of dyes which otherwise would not be usable for dyeing anodized aluminum; it provides for maximum coloration yield; it permits the use of mixtures of dyes without bleed of one or more components; it minimizes contamination of sealing baths; and it enables a printer to main tain sharp lines and high contrast on aluminum surfaces which are printed and then sealed.

While the present invention has been described in conjunction with advantageous embodiments, those skilled in the art will recognize that modifications and variations may be resorted to without departing from the spirit and scope of the invention. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

I claim:

1. A process for coloring and sealing anodized aluminum comprising introducing a dye insoluble in water into the pores of an unsealed anodized coating on an aluminum surface and thereafter sealing said surface by contacting said surface with a relatively concentrated aqueous solution of a neutral salt maintained at a temperature of at least about 90 C.

2. A process for coloring and sealing anodized aluminum comprising introducing a dye insoluble in water into the pores of an unsealed anodized coating on an aluminum surface and thereafter sealing said surface by contacting said surface with 'an aqueous solution containing at least about 15 percent by weight of sodium sulfate maintained at its boiling point.

' 3. A process as in claim 2 wherein the water-insoluble dye is introduced into the pores of the anodized coating by contacting said coating with a solution of said dye in an organic solvent.

4. A process as in claim 2 wherein the water-insoluble dye is introduced into the pores of the anodized coating by contacting said coating with vaporized dye.

5. A process as in claim 2 wherein the water-insoluble dye is introduced into the pores of the anodized coating by contacting said coating with a printing ink containing said dye.

THe essentially peculiarity for the production of artificial speech. 

2. A process for coloring and sealing anodized aluminum comprising introducing a dye insoluble in water into the pores of an unsealed anodized coating on an aluminum surface and thereafter sealing said surface by contacting said surface with an aqueous solution containing at least about 15 percent by weight of sodium sulfate maintained at its boiling point.
 3. A process as in claim 2 wherein the water-insoluble dye is introduced into the pores of the anodized coating by contacting said coating with a solution of said dye in an organic solvent.
 4. A process as in claim 2 wherein the water-insoluble dye is introduced into the pores of the anodized coating by contacting said coating with vaporized dye.
 5. A process as in claim 2 wherein the water-insoluble dye is introduced into the pores of the anodized coating by contacting said coating with a printing ink containing said dye. 